Those skilled in the art are familiar with terrain awareness and warning systems (TAWS). The purpose of these systems is to generate an alert when the aircraft is in a hazardous situation where the operational margins are no longer observed. The TAWSs, in the form of a stand-alone computer or incorporated with the TCAS (traffic collision avoidance system) and WXR (weather band X radar) functions in an ISS (integrated surveillance system), fulfil a primary anti-collision surveillance function (“safety net”) with the terrain and are designed to transmit sound alerts upon an exceptional approach of the relief enabling the crew to react by engaging a vertical resource before it is too late. To do this, the TAWS systems, decoupled from navigation systems, proceed in two ways. They periodically compare the theoretical trajectory that the aircraft would describe in a resource and compare it to a cross section of the terrain and to the obstacles being flown over obtained from a digital world or local terrain model installed on the computer. Or there are some TAWSs that also incorporate “reactive modes” which, by periodically comparing certain of the current parameters of the craft, for example the radio-altitude and the vertical speed, determine through different charts whether the current situation of the aircraft is a normal situation or whether it is potentially dangerous. In the latter case, an alert, limited to an oral message, is generated to inform the crew. The availability of a model of the terrain allows functions that make it possible to improve the perception of the situation on the part of the crew. Among these, the alert lines are designed to delimit the areas of terrain for which a TAWS alert is likely to appear. As for the alert areas, these show the areas provoking a TAWS alert.
Numerous patent documents describe this type of system. Among them can be cited the patent “EP0 565399 B1” describing all the basic concepts of the TAWSs and the patent application “US 2003/0107499A1” describing a device for displaying areas of terrain with risk and capable of provoking a TAWS alert.
The functions handled by a terrain anti-collision device show limitations. Existing secondary functions of TAWS systems propose a simplified cartographic representation dissociated from the concept of local safety altitude or from values associated with the emergency procedures. Moreover, the safety and operational objectives of such equipment do not destine them to fulfil navigation aid functions. Furthermore, the alert function triggers a message addressed to the crew when a certain safety threshold is crossed. It does not allow the difference relative to this threshold to be calculated or represented.
Other terrain anti-collision devices propose, in the current prototype state, a function for automating the vertical resource in the case of a terrain alert. However, these devices have the drawback of not taking into account the instantaneous vertical climb capabilities of the aircraft, but use default climb rates, a climb angle of 6° for example. Consequently, the proposed avoidance manoeuvre is not guaranteed according to the actual weight or the local temperature. Such an approach is incompatible with the integrity objectives of navigation systems.
The onboard flight management system (FMS) is the computer that determines the geometry of the profile in four dimensions, the three-dimensional space and the time parameter, and which sends to the pilot or to the automatic pilot guidance setpoints to follow this profile. The system calculates the lateral trajectory and the vertical profile, which minimizes the cost, from aeroplane data such as, for example, the weight, the flight plan and the range of cruising levels. The FMS cannot be used as a navigation aid system in the face of obstacles because, for the creation of a flight plan, the system does not check the intersection of the proposed trajectory with the relief and does not have a digital terrain model available with which to perform the calculations to maintain separation from the predicted trajectory with the relief.